1. Field of the Invention
The present invention relates to a feedback controller for performing a feedback control through detecting a driving state of a driving unit, a digital filter device which is preferably applicable to the feedback controller, and a storage device having a head for at least reproducing information stored in an information storage medium.
2. Description of the Related Art
Hitherto, there is widely used in various fields a feedback controller in which a feedback signal is generated through detecting a driving state of a driving unit, an arithmetic processing such as a phase compensation for a stabilization of a control system is performed, and a driving control of the driving unit is performed in accordance with the feedback signal after the arithmetic processing.
According to the feedback controller as mentioned above, in order to enhance the tracking efficiency, there is adopted such a scheme that a phase-lag compensation is established, a feedback gain is increased to establish a high frequency band.
It is known that when it is intended to increase a gain to a high frequency by the phase-lag compensation, a phase cross frequency is increased, and then the gain margin and the phase margin are decreased, so that a stability of a control system is damaged.
Therefore, there is a need to establish a high frequency band through increasing a gain cross frequency to some extent. However, in the even that in order to increase the gain cross frequency, the feedback gain is increased, there is a need to establish a phase-lead compensation over a high frequency so that a control system is stabilized in the high frequency. In this case, there is a possibility that the stability of the control system is damaged by a high-order resonance of a movable mechanism unit.
In order to solve this problem, there is adopted a scheme that a notch filter or a low pass filter is applied to a high-order resonance frequency. However, in case of a digital filter used in a digital control using a microprocessor unit (MPU), such as a digital signal processor (DSP) and the like which are used for the purpose of establishing low cost and high-performance, the digital filter does not sufficiently operate at a frequency close to the Nyquist frequency which is xc2xd of the sampling frequency. Therefore, in the event that a high-order resonance frequency of the movable mechanism unit exists at a frequency band close to the Nyquist frequency, it is difficult to ensure a sufficient stability of the control system.
The above-mentioned problem will be described referring to by way of example a feedback controller adopted in an optical storage device for accessing an optical disk. As the optical disk, for example, a phase change optical disk and a magneto optical disk exist. Here, typically, an optical storage device for accessing the magneto optical disk will be considered.
Hereinafter, first, a guide line of the optical storage device will be described, and then problems of the feedback control system will be described.
FIG. 1 is a perspective view of an optical storage device.
A spindle motor 101 for driving an optical disk 200 is fixed on a drive base 100, for example, made of an aluminum. Further, on the drive base 100, there are provided a movable mechanism unit 110 having an objective lens 111 and a magnetic coil 112, and a magnetic circuit 121 having a permanent magnet disposed in such a manner that the movable mechanism unit 110 is sandwiched by the permanent magnet. The magnetic circuit 121 having the permanent magnet and the magnetic coil 112 constitutes a voice coil motor (VCM). As a current is supplied to the magnetic coil 112, the movable mechanism unit 110 moves in a direction of an arrow A-Axe2x80x2 by an interaction of the current flowing through the magnetic coil 112 and the magnetic circuit. A laser beam is applied from a fixed optical unit 130 to the objective lens 111. The laser beam is emitted from the objective lens 111 so that an optical spot is projected onto an optical disk 200 and reflected therefrom. The reflected light is returned through the objective lens 111 to the fixed optical unit 130, so that information stored in the optical disk 200 is picked up.
FIG. 2 is a schematic construction view of an optical system of an optical storage device.
A laser beam emitted from a semiconductor laser 131 passes through a collimator lens 132 and a polarization beam splitter 133, and reflects-on a reflecting mirror 113, and further passes through the objective lens 111, and finally be condensed on the optical disk 200.
Here, only the reflecting mirror 113 other than the objective lens 111 is mounted on the movable mechanism unit 110, and other all optical elements constitute the fixed optical unit 130.
A signal light reflected on the optical disk 200, which carries information recorded on the optical disk 200, passes through the objective lens 111, reflects on the reflecting mirror 113, enters the polarization beam splitter 133, and goes to a beam splitter 134 side. An incident light to the beam splitter 134 is divided into two parts one of which enters Wollaston prism 135 whereby the light is separated in accordance with the polarization direction. And the light thus separated enters via a lens 136 a photo detector 137 for picking up information recorded on the optical disk 200.
On the other hand, another of the two parts of light divided by the beam splitter 134 enters via a lens 138 a beam splitter 139 wherein the light is further divided into two parts one of which enters a photo detector 140 for a tracking error detection, and another enters a wedge prism 141 wherein a light beam is further divided into two parts and is projected onto a photo detector 142 for a focus error detection.
FIG. 3 is a block diagram of a feedback controller for driving the movable mechanism unit 110 of the optical storage device.
A position of the movable mechanism unit 110 is detected by a positional sensor 150. The positional sensor 150 comprises the photo detector 140 for tracking an error detection, as shown in FIG. 2, and a signal processing circuit (not illustrated) for processing signals obtained by the photo detector 140. A detection signal obtained by the positional sensor 150 is fed to a differential circuit 152 in which a difference between the detection signal and a target value signal representative of a position of the movable mechanism unit 110, outputted from a target value generating circuit 151, is calculated to generate an error signal. The error signal thus generated is attenuated in high frequency component by an anti-aliasing filter 153 for suppressing a frequency component exceeding Nyquist frequency which is the half of a sampling frequency of an A/D converter 154. The A/D converter 154 converts the error signal thus obtained into a digital signal. A signal outputted from the A/D converter 154 is fed to a phase compensation filter 155 wherein the signal is subjected to a phase compensation processing so as to ensure a stability of a control system through a digital operation. A signal subjected to the phase compensation processing by the phase compensation filter 155 is fed to a driving circuit 156 in the form of a control signal to control the movable mechanism unit 110 to a target position. Incidentally, it is acceptable that the driving circuit 156 receives a digital control signal, or alternatively it is acceptable that the driving circuit 156 receives an analog control signal in such a manner that the output of the phase compensation filter 155 is converted into the analog control signal by a D/A conversion.
The driving circuit 156 supplies a driving signal (here a current signal) to an electromagnetic coil 112 (cf. FIG. 1) of the movable mechanism unit 110 in accordance with the entered control signal, and whereby the movable mechanism unit 110 is controlled to a target position.
FIGS. 4(A) and 4(B) are views showing an example of a frequency characteristic of a displacement of the movable mechanism unit 110 to a current supplied from the driving circuit 156. FIG. 4(A) shows a gain characteristic, and FIG. 4(B) shows a phase characteristic.
Here, the sampling frequency 55 kHz is used. Therefore, the Nyquist frequency is 27.5 kHz which is xc2xd of the sampling frequency.
The anti-aliasing filter 153 can sufficiently attenuate a high-order of resonance and an electric noise component exceeding 30 kHz. In the event that a cut-off frequency of the anti-aliasing filter 153 is set up to a further low frequency side, it would have an effect on a frequency band related to a control stability in a phase delay. Consequently, it is impossible to set up the cut-off frequency to the lower frequency side than 30 kHz.
At the lower frequency side than 30 kHz, as shown in FIG. 4(A), there exists three high-order of resonance of frequencies A, B and C, where A is about 16 kHz, B is about 22 kHz, and C is about 27.5 kHz which is substantially the same as the Nyquist frequency.
These high-order of resonance of frequencies cause the control system to be insecure.
FIGS. 5(A) and 5(B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system.
An effect of the differentiation appears up to the vicinity of the Nyquist frequency to ensure a phase margin at the gain close frequency (a frequency at the point where a gain curve crosses 0 dB), and the gain rises. This filter characteristic shows a driving sensitivity to the input error signal, and as seen from FIG. 5(A), the driving sensitivity offers the highest value to an input of the Nyquist frequency (about 27.5 kHz).
FIGS. 6(A) and 6(B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system, in a similar fashion to that of FIGS. 5(A) and 5(B), and in addition a notch filter of 27.5 kHz is disposed in the feedback system.
As will be understood from FIGS. 6(A) and 6(B), the notch filter almost has no effect at the frequency band (high-order of resonance C) near the Nyquist frequency.
FIG. 7 is a view showing a signal wave form of an error signal outputted from the differential circuit 152 when the noise of 27.5 kHz is added to the feedback control system, and a driving signal outputted from the driving circuit 156. FIG. 8 is a view showing a signal wave form in which a part of the signals shown in FIG. 7 is enlarged on a time basis.
As shown in FIGS. 7 and 8, it will be understood that both the error signal and the driving signal vibrate at 27.5 kHz. FIGS. 7 and 8 show a result when the noise of 27.5 kHz is added intentionally to the feedback control system for the purpose of a confirmation of the stability of the control system. However, it is poor in the stability of the control system, and in the event that the movable mechanism unit 110 is considered in connection with an unevenness on production, there is a high possibility that a defective unit in which the movable mechanism unit 110 is excited unstably appears.
In the event that a high-order resonance frequency of the movable mechanism unit exists at the frequency band near the Nyquist frequency, as mentioned above, in order to ensure a sufficient stability of the control system, hitherto, there is a need to increase the sampling frequency, or alternatively to change a design of the movable mechanism unit so as to increase the high-order resonance frequency.
However, in order to increase the sampling frequency, there is a need that the A/D converter and other digital signal processing system are operated at high speed. In many cases, it is not permitted in view of the cost of manufacturing. On the other hand, there is a limit in a point that the high-order resonance frequency of the movable mechanism unit is increased, and also with respect to the change of the design, in many cases, it is difficult through a change of the design to expect that the high-order resonance frequency of the movable mechanism unit is increased to a sufficiently high frequency. Further, in some case, it is difficult to change the design per se.
FIGS. 9(A)-(C) are explanatory views useful for understanding another problem involved in the vibration of the driving signal.
FIG. 9(A) shows a signal waveform of a driving signal which is essentially necessary. FIG. 9(B) shows a signal waveform of a signal in which a high frequency noise is superposed on the essentially necessary driving signal. FIG. 9(C) shows a signal waveform of a signal in which a further large high frequency noise is superposed on the essentially necessary driving signal and thereby reaching a saturation level.
As the high frequency noise is superposed on the driving signal, even if the high-order resonance of the movable mechanism unit does not exist on the frequency, the saturation of the high frequency noise brings about disappearance or attenuation of the essentially necessary driving signal as shown in FIG. 9(C). Thus, there is a possibility that a normal feedback control is not performed.
In view of the foregoing, it is an object of the present invention to provide a feedback controller capable of ensuring a sufficient stability, a digital filter which is preferably applicable to the feedback controller, and a storage device having a head for at least reproducing information stored in an information storage medium, and being capable of driving the head stably and with a sufficient tracking performance.
To achieve the above-mentioned objects, the present invention provides a feedback controller wherein a driving state of a driving unit is detected to generate feedback signals so that a driving control for the driving unit is performed, said feedback controller comprising:
a sampling unit for sampling on a digital basis the feedback signals at a predetermined first period;
a filter input unit for sequentially holding the feedback signals sequentially transmitted from said sampling unit at the first period, and for sequentially outputting the transmitted feedback signals or held feedback signals at a second period which is shorter than the first period;
a digital filter for applying a filtering processing to the feedback signals outputted from said filter input unit; and
a filter output unit for sequentially outputting the feedback signals subjected to the filtering processing by said digital filter, while thinning the same, at the first period.
In the above-mentioned feedback controller, it is acceptable that the feedback signals, which are generated upon detection of a driving state of the driving unit, are signals per se derived through a sensor for detecting the driving state, or alternatively it is acceptable that the feedback signals are signals processed involved in the signals derived from the sensor, for example, error signals representative of differences between the signals derived from the sensor and a target positional signal.
According to the present invention, the filter input unit generates the feedback signals at the second period which is shorter than the first period (a sampling period), and the digital filter applies a filtering processing to the feedback signals at the second period. This feature makes it possible to expect a sufficient filtering effect as to a frequency near for example a Nyquist frequency. Further, according to the present invention, the filter output unit restores the signals outputted from the digital filter to signals of the original first period. Thus, it is sufficient that only the portion of the digital filter is subjected to a high frequency arithmetic operation. Therefore, as compared with a case where a sampling frequency in the sampling unit is increased per se, there is no need to increase an operating speed for a digital arithmetic operation processing for the sampling unit and other than the portion of the digital filter. Thus it is possible to greatly suppress a manufacturing cost.
In the feedback controller according to the present invention as mentioned above, it is preferable that said filter input unit sequentially outputs the transmitted feedback signals or the held feedback signals at the second period which is 1/N (where N=integer) of the first period.
Adoption of the period which is 1/N (where N=integer) of the first period makes it possible to simplify an arrangement of the filter input unit and the filter output unit.
In the feedback controller according to the present invention as mentioned above, it is acceptable that said digital filter is a notch filter for eliminating or attenuating a specific frequency component, for example, a frequency component which is the same as a Nyquist frequency.
In the feedback controller according to the present invention as mentioned above, while the feedback controller according to the present invention is not restricted in a use, it is preferable that the feedback controller controls a detection head for picking up information stored in an information recording medium in such a manner that the detection head comes close to the information recording medium and moves, and more particularly the feedback controller controls a movement of the detection head.
Here, it is acceptable that the information recording medium is an optical disk or alternatively a magnetic disk. And thus, it is acceptable that the detection head is the optical head as show in FIGS. 1 and 2, or alternatively a magnetic head for electro-magnetically picking up information.
Further, according to the present invention, there is provided a digital filter device comprising:
a filter input unit for sequentially holding digital signals, which are sequentially transmitted at a predetermined first period, at the first period, and for sequentially outputting the transmitted digital signals or held digital signals at a second period which is shorter than the first period;
a digital filter for applying a filtering processing to the digital signals outputted from said filter input unit; and
a filter output unit for sequentially outputting the digital signals subjected to the filtering processing by said digital filter, while thinning the same, at the first period.
According to the digital filter of the present invention as mentioned above, it is possible to have an effective filtering function on the frequency near the Nyquist frequency.
Furthermore, according to the present invention, there is provided a storage device having a head for at least reproducing information stored in an information storage medium, said storage device comprising:
a driving unit for moving and controlling said head;
a feedback signal generating unit for detecting a position of said head and generating feedback signals to be fed to said driving unit on a feedback basis;
a sampling unit for sampling on a digital basis the feedback signals at a predetermined first period;
a filter input unit for sequentially holding the feedback signals sequentially transmitted from said sampling unit at the first period, and for sequentially outputting the transmitted feedback signals or held feedback signals at a second period which is shorter than the first period;
a digital filter for applying a filtering processing to the feedback signals outputted from said filter input unit; and
a filter output unit for sequentially outputting the feedback signals subjected to the filtering processing by said digital filter, while thinning the same, at the first period.
In the storage device according to the present invention as mentioned above, it is acceptable that said driving unit is a track driving unit for moving said head in a track direction, or alternatively it is acceptable that said driving unit is a focus driving unit for moving said head in a focus direction.
As seen in the storage device having a head for reproducing information stored in, for example, an optical disk, the driving in the track direction is to perform a double servo by both the carriage and the track actuator. In some cases, there is provided a single servo in which only a carriage has a driving unit. The storage device according to the present invention is applicable to both the single servo and the double servo. And further, in case of the double servo, the storage device according to the present invention is applicable to either one or both of the carriage and the track actuator.
According to the storage device of the present invention, it is possible to improve the follow-up performance of the head. That is, an application of the storage device to a track driving unit for moving a head in a track direction makes it possible to improve a track follow-up performance of the head, since the head moves promptly, and thus a control band is expanded. Further, an application of the storage device to a focus driving unit for moving the head in a focus direction makes it possible to improve the follow-up performance of the head in the focus direction, and whereby the head promptly responds to being out of focus.